When booting MorphOS from OF, you've got to pass the bootdevice=dh2 (for example, must be a drive name, not a logical name). You could even create 2 HFS boot partitions, modify the bootinfo.txt to include said bootdevice argument so that you could use Apple's boot menu. That'll suck a bit since you mention NVidia as that's where the boot menu will show up.
I do advise starting by messing with OF prompts rather than modifying bootinfo.txt. Mind that you need to call "hfsssetmacboot boot:bootinfo.txt" from a shell to re-bless the file after editing of OpenFirmware won't read it.
